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In page Jigsaw puzzle:

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Otherwise, all or most pieces of a modern jigsaw puzzle interlock by means of rounded tabs (interjambs) and indentations (called "blanks") on adjacent sides. The pieces are normally four-sided and may be uniform in appearance except for the edges and corners. Some puzzles are termed "fully interlocking", which means that a group of assembled pieces fit together tightly enough to be moved without falling apart; sometimes the connection is tight enough that a solved section will remain attached when lifted by one piece. Uniformly shaped fully interlocking puzzles, sometimes called "Japanese Style", are more difficult because pieces are hard to tell apart.[citation needed]